Recently, in order to prevent an excess loading on large vehicles, such as trucks, it has been proposed to incorporate a load measuring apparatus directly in the vehicle body to enable a driver or loading people to easily determine a load.
For example, a vehicle load measuring apparatus was proposed in Japanese Utility Model Laid-Open Publication No. 6-69759, such as shown in FIG. 5. Here, a suspension structure of a large vehicle, having leaf springs 26a and 26b, has load sensors of a magnetostrictive type sensor or a distortion sensor mounted on members which receive a load given by a weight of loaded goods. More specifically, at the side of front wheels 25, sensors 7a.sub.1 and 7a.sub.2 are attached to a shackle pin 34 connecting a bracket 31 and a shackle 32 as shown in FIG. 6. Further, at the side of rear wheels 20, sensors 7b.sub.L, 7b.sub.R are attached to a trunnion shaft 3 supported by a trunnion bracket 2A load applied to these members can be detected by these sensors, to measure the load and/or the vehicle weight.
Specifically, as shown in FIG. 5 and FIG. 6, the load at the side of the front wheels 25 is detected in such a manner that an axial hole 6 is provided along an axial direction on a center line of the shackle pin 34 connecting the suspension (leaf spring) 26a of the vehicle and the bracket 31 on the side of a loading platform frame 1 by way of a bush 33. A pair of load sensors 7a.sub.1, 7a.sub.2 are disposed in the axial hole 6 to detect distortion of the shackle pin 34 which is deformed in proportion to the load above the spring of the vehicle at the side of front wheels 25.
The pair of load sensors 7a.sub.1, 7a.sub.2 are respectively mounted on the right and left shackle pins 34 each disposed at right side and left side of the vehicle, and accordingly in this conventional apparatus, a sum of a front right axle sensor output f.sub.FR and a front left axle sensor output f.sub.FL are respectively a sum of outputs of the pair of load sensors 7a.sub.1, 7a.sub.2 mounted on the right and left shackle pins 34.
On the other hand, the load at the side of rear wheels 20 is detected by the load sensors 7b.sub.L, 7b.sub.R. As shown in FIG. 5 and FIG. 7, axial holes 6', 6' are provided along an axial direction on a central line of a horizontal axle 3a of the trunnion shaft 3, which is engaged with the trunnion bracket 2 attached to the loading platform frame 1, and the load sensors 7b.sub.L and 7b.sub.R are disposed in the axial holes 6', 6' to detect distortion of the trunnion shaft 3, which is deformed in proportion to the load above the spring of the vehicle at the side of rear wheels 20. Output signals respectively provided by the sensors 7a.sub.1, 7a.sub.2, 7b.sub.L, and 7b.sub.R (collectively, 7a, 7b) are amplified by amplifiers 41a, 41b to obtain the load above the springs of the vehicle. Then, computation process of adding a load below the springs of the vehicle is carried out by a controller 40, to measure the vehicle load and/or the load, and, if necessary, the measured value is displayed on a display device 42.
In the foregoing conventional apparatus, a sensor output of the load sensor 7b.sub.L is a rear left axle sensor output f.sub.RL, and a sensor output of the load sensor 7b.sub.R is a rear right axle sensor output f.sub.RR.
Specifically, the controller 40 performs a predetermined computation process based on the detected signals, representative of the vehicle load, a switching signal from each switch which will be described hereinafter, and an input signal, such as, a signal representing a vehicle speed detected and input by a vehicle speed sensor 47. The resulted signals representative of the load and the total weight of the vehicle are sent to the display device 42 in a driver's cabin.
However, with the above-described conventional apparatus, the sensors 7a, 7b are mounted on and through the shackle pin 34 and the trunnion shaft 3 which receive the weight of the vehicle. Since these members need to receive the weight of the vehicle, they are disposed in the vicinity of various heat sources, such as an engine, an exhaust pipe and the like, located at the lower portion of the vehicle body. These heats are transferred to the shackle pin 34 and the trunnion shaft 3. Thus, such heat conduction before and after running of the vehicle causes changes in a distortion factor despite the fact that the weight of load is the same, and results in causing changes in a computed load value, which becomes an error factor. And, even with a layout of positioning the heat sources remote from the shackle pin 34 and the trunnion shaft 3, there will be cases where the vehicle is subjected to the radiant heat from the ground surfaces or the atmospheric temperature, while the vehicle is being stopped or running, and this will cause changes in the distortion factor despite the fact that the weight of load is the same, similar to the above-described case, and as a result, there will be changes in the computed load value.
Such phenomenon of having fluctuations in the computed load value due to temperature changes, despite a fixed weight of load, occurs not only with the use of the magnetostrictive type sensors or the distortion sensors as in the conventional art, but also with other types of load sensors (for example, a sensor for sensing an inner pressure of an air spring).
In view of the above-described drawbacks of the prior art, it is an object of the present invention to provide a vehicle load measuring apparatus capable of making an accurate measurement of a load weight or a vehicle weight, even when there are temperature changes.